In recent years there has been an increase in the demand for a non-toxic replacement for the traditional lead shot pellets used in shotguns. The use of toxic lead shot by hunters leads to the poisoning of many natural habitats, particularly the lakes and ponds where ducks, geese and other waterfowl are found. Presently, the only approved alternatives to lead shot are steel shot and bismuth-tin alloy, which are not toxic like lead. Unfortunately, steel shot has significant drawbacks compared to lead shot. Steel shot is much less dense than lead shot and therefore slows down much more rapidly than lead shot when fired. This makes hunting with steel shot less accurate because hunters must "lead" a moving target by a greater amount when using steel shot. Furthermore, because steel shot is so much lighter than lead shot, it causes far less shock and injury to the hunter's quarry. Rather than dying quickly after impact, the wounded animal will frequently run or fly a great distance before bleeding to death. Additionally, steel shot is more damaging to shotgun barrels because steel is harder than lead.
A more suitable replacement for lead is bismuth shot, in particular bismuth alloy shot. Bismuth may be alloyed with other metals, such as tin, to thereby produce bismuth alloy shot that is very nearly the same density as lead. Bismuth alloys are soft like lead and non-toxic. For these reasons, bismuth alloy shot does not damage shotgun barrels the way steel shot does and, because of its comparable density, has the accuracy and killing power of lead shot.
U.S. Pat. No. 4,949,644 to Brown discloses a non-toxic wildlife shot pellet for use in shotgun shells that is formed from bismuth or bismuth alloys. U.S. Pat. No. 5,279,787 to Oltrogge discloses methods of manufacturing and compositions of non-toxic projectiles containing bismuth, wherein the projectiles are made of high melting point powders mixed with molten metals of a low melting point to thereby produce a sintered metal projectile. These two prior art references are hereby incorporated by reference.
The principal drawback to using bismuth alloy shot is producing it economically in commercial quantities so as to be a viable replacement for lead shot. Unlike lead and steel, bismuth is highly crystalline and extremely nonductile. Bismuth also has a very low melting temperature and expands upon cooling. These metallurgical properties make it difficult or impossible to economically produce bismuth shot by conventional methods.
Normal shot sizes for hunting and shooting loads range from 0.008 inches to 0.33 inches. (No. 9 shot to No. 4 Buckshot) Conventional methods of making smaller sizes of shot (i.e., No. 9 to No. 6) are the drop method and the Bleimeister method (sometimes called the "short drop" method).
In the drop method, molten lead is poured through a screen at the top of a shot tower. The "screened" lead chills and solidifies into spheres as it falls through the air before landing, typically, in a tank of cold water.
In the Bleimeister method, molten lead is pumped through a pair of arms having small orifices along their bottom surfaces. The arms are positioned about 4 inches above the surface of almost boiling water. Droplets of molten metal fall through the small orifices in the bottom of the arms into the water and then roll along the surface of inclined plane pine boards to form spherical beads of molten lead. The spherical beads of molten lead harden as they roll and then drop to the bottom of the water tank, cooling on the way down.
Bismuth is highly crystalline in nature and resists forming a sphere. Bismuth also expands when cooled and has a high degree of heat retention, which prevents it from cooling quickly enough to be made using the drop method. These properties make it impossible to manufacture large bismuth alloy shot pellets using the drop method and the Bleimeister method. It is also not possible to manufacture lead or steel shot greater than size No. 5 (0.12") using either of these methods.
Large lead shot is conventionally made by a process that extrudes lead wire which is then flattened into an oval ribbon by a pair of rollers. The flattened oval ribbon is run through a pair of die wheels that punch out spheres of the appropriate size. Round steel shot is made by drawing a wire and snipping it into short segments using a header machine. The header machine then grinds the short chunks of steel wire into spheres.
Because of the nonductility of bismuth and bismuth alloys, it is not commercially feasible to make bismuth alloys into wire. Therefore, neither the header machine process nor the ribbon tape process may be used to make bismuth shot because both processes require that the bismuth alloy first be extruded into a wire.
The shortcomings of the previously discussed methods of manufacturing bismuth alloy shot leads to the conclusion that one method of manufacturing bismuth alloy shot that has a chance to produce shot economically for commercial use is to use high-speed die casting machinery capable of producing large quantities of shot in a short time frame. The manufacturing cost of bismuth shot is very critical due to the greater cost of raw bismuth compared to lead. The cost of bismuth is typically ten times that of conventional materials such as steel and lead. Thus, any die casting process must yield a high product to waste ratio.
Analysis revealed that manufacturing costs of $5 a pound on top of raw material costs would probably result in shot shells that were too expensive to be commercially acceptable to purchasers. Given the purchase price (or lease cost) of commercially available high-speed die casters and the operating costs associated with the die casters, it was determined that a minimum production level of at least a hundred pounds per hour of bismuth alloy shot had to be attained. Unfortunately, there were no die-blocks commercially available for producing shot made from bismuth alloy. The initial die-blocks custom fabricated for the inventors using traditional die design techniques failed to produce an acceptable number of shot pellets per casting in order to attain one hundred pounds of shot pellets per hour.
For example, number 4 shot requires approximately 1600 pellets per pound, at a rate of 100 pounds per hour, to be commercially viable. Traditional mold design techniques used to cast small parts cannot attain these numbers because they have an unacceptably high ratio of sprue and runner material to shot pellets. For example, in most cases, approximately 70% of the metal injected into the die block forms sprues and runners to which the shot pellets are attached. These sprues and runners are waste metal that is remelted in the melting pot and represent a cost of approximately $3.11 per pound of waste per casting, exclusive of any other manufacturing costs.
Conventional high speed die casters and die blocks were also found to be unsuitable for casting bismuth alloy shot pellets because the heat retention of the bismuth increased the cycle time of the machines, minimizing production rates. Also, the expansion of bismuth when cooled was in part responsible for slowing down the flow of the material through the die block.
There is therefore a need for a method of quickly and economically manufacturing shot made from bismuth and bismuth alloys in various sizes.